Global sinusoidal seasonality in precipitation isotopes

Quantifying seasonal variations in precipitation delta H-2 and delta O-18 is important for many stable isotope applications, including inferring plant water sources and streamflow ages. Our objective is to develop a data product that concisely quantifies the seasonality of stable isotope ratios in precipitation. We fit sine curves defined by amplitude, phase, and offset parameters to quantify annual precipitation isotope cycles at 653 meteorological stations on all seven continents. At most of these stations, including in tropical and subtropical regions, sine curves can represent the seasonal cycles in precipitation isotopes. Additionally, the amplitude, phase, and offset parameters of these sine curves correlate with site climatic and geographic characteristics. Multiple linear regression models based on these site characteristics capture most of the global variation in precipitation isotope amplitudes and offsets; while phase values were not well predicted by regression models globally, they were captured by zonal (0-30 degrees and 30-90 degrees) regressions, which were then used to produce global maps. These global maps of sinusoidal seasonality in precipitation isotopes based on regression models were adjusted for the residual spatial variations that were not captured by the regression models. The resulting mean prediction errors were 0.49 parts per thousand for delta O-18 amplitude, 0.73 parts per thousand for delta O-18 offset (and 4.0 parts per thousand and 7.4 parts per thousand for delta H-2 amplitude and offset), 8 d for phase values at latitudes outside of 30 degrees, and 20 d for phase values at latitudes inside of 30 degrees. We make the gridded global maps of precipitation delta H-2 and delta O-18 seasonality publicly available. We also make tabulated site data and fitted sine curve parameters available to support the development of regionally calibrated models, which will often be more accurate than our global model for regionally specific studies.